If you're interested, I've written about why they're forced to do so at [1] and [2]. (And the reason I'm following it so closely is because I'm trying to fix it - if you want to follow that, see [3].)
[1] https://medium.com/flockademic/the-vicious-cycle-of-scholarl...
[2] https://medium.com/flockademic/the-ridiculous-number-that-ca...
Indexed preprints make a whole lot of sense, if they'll let you do it.
That said, there are often funds available from e.g. the library, your funder, or even nationwide funds such as in the Netherlands and I think the UK, that will cover the costs of making articles available as open access. It's pretty much a no-brainer to use them for researchers.
Not in my experience or at least no exclusively. A lot of researchers are paying for open access. Sure they all grumble about it and think it's a stupid system, but since it's neither their money nor in many cases their decision, they still do it.
Elsevier lost when they agreed to allow people (mostly just some people in math and physics back then) to post preprints online.